In 10 G-EPON (10 Gb/s Ethernet® Passive Optical Network) realizing a communication speed of 10 gigabits/second in a public network including optical fibers, an optical signal from a subscriber device is detected by a light reception element such as an APD (Avalanche Photo Diode), and a preamplifier such as a TIA (transimpedance amplifier) is provided in a terminal device in order to amplify a current output by this light reception element.
In addition, reduction in size of optical transmitters and receivers has been demanded, and when a transmitter and a receiver are adjacent to each other for reduction in size, deterioration in reception sensitivity due to crosstalk of a signal from the transmitter to the receiver gives rise to a problem. In particular, in 10 G-EPON, the problem of crosstalk is more noticeable than GE-PON which is EPON realizing a communication speed of 1 gigabit/second. Here, influence by crosstalk on a circuit supplying a bias voltage to an APD is great, and in order to lower crosstalk, it is effective to arrange a bypass capacitor in close vicinity to the APD.
U.S. Pat. No. 7,414,234 (PTD 1) discloses a strength detection (RSSI: Received Signal Strength Indicator) circuit for an optical receiver including a light reception element as below. Namely, an output current of the light reception element is mirrored by a current mirror circuit, and the mirrored current is used for monitoring. Specifically, a capacitor is charged with the mirrored current and the time when a charging voltage exceeds a threshold value is counted.
In such a configuration that a current mirror circuit is employed in the RSSI circuit as above, responsiveness to an optical burst signal becomes an issue. Namely, in the current mirror circuit, while no signal or a weak signal is received, a voltage across the base and the emitter of a transistor lowers and a resistance component (dV/dI) increases, which in turn delays burst response from reception of no signal and a weak signal and burst response to reception of a weak signal. In particular, in a case that a bypass capacitor is arranged on a side of the APD to which a bias voltage is applied, a low-pass filter is formed by the bypass capacitor and the resistance component of the transistor and burst response is delayed.
“Current Sense Circuit Collection,” Linear Technology Application Note 105, December 2005 (NPD 1) discloses a circuit in which a current to be monitored is fed through a sense resistor and a voltage across opposing ends of the sense resistor is received by a differential amplifier.